A parallel MPI+OpenMP+OpenCL algorithm for hybrid supercomputations of incompressible flows

The work is devoted to the development of efficient parallel algorithms for large-scale simulations of incompressible flows on hybrid supercomputers based on massively-parallel accelerators. The governing equations are discretized using a high-order finite-volume scheme for Cartesian staggered meshes with the only restriction that, at least, one direction is periodic. Its "classical" MPI + OpenMP parallel implementation for CPUs was designed to scale till 100,000 CPU cores. The new hybrid algorithm is developed on a base of a multi-level parallel model that exploits several layers of parallelism of a modern hybrid supercomputer. In this model, MPI and OpenMP are used on the first two levels to couple nodes of a supercomputer and to engage its CPU cores. Then, computing accelerators are further used by means of the hardware independent OpenCL computing standard. In this way, the implementation is adapted to a general computing model with central processors and math co-processors. In this paper the work is focused on adapting the basic operations of the algorithm to architectures of Graphics Processing Units (GPU) without considering the multi-GPU communication scheme. Technology of porting the code to OpenCL is described, certain optimization approaches are presented and relevant performance results obtaining up to 80–90 GFLOPS on a GPU accelerator are demonstrated. Moreover, the experience with different GPU architectures is summarized and a comparison based on the particular application is given for AMD and NVIDIA GPUs as well as for CUDA and OpenCL frameworks.